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Author Version of : Marine Environmental Research, vol.157; 2020; Article no: 104926 

Variation in phytoplankton community and its implication to dimethylsulphide production at a coastal station off Goa, India

Bhagyashri R. Naik1,2, Mangesh Gauns1,*, Kausar Bepari1,3 , Hema Uskaikar1 and Damodar M. Shenoy1

1 CSIR-National Institute of Oceanography, Dona Paula, Goa, India. 2 School of Earth, Ocean and Atmospheric Sciences, Goa University, Taleigao Plateau, Goa, India

3 Centre for Marine Living Resources and Ecology, Kochi, Kerala, India. * To whom correspondence should be addressed: e mail: gmangesh@nio.org

Highlight: Prevailing seasonal low oxygen condition along the west coast of India supports DMS build-up, which is important from the viewpoint of global climate change

Abstract

Seasonal hypoxia/suboxia (at times anoxia) towards the end of Southwest monsoon (SWM; June to September) at the coastal time series site off Goa, West coast of India was found to influence the dynamics of phytoplankton biomass, community structure and production of climatically active gas, dimethylsulphide (DMS). In this diatom dominated study region, high DMS production in the subsurface waters during late SWM might possible be attributed to the stress experienced by micro- and macro-algae from the prevailing low oxygen subsurface waters through different pathways specifically believed to be via methylation pathway (see Schafer et al., 2010). Based on laboratory experiments, we hypothesize presence of floating seaweeds mostly Sargassum species washed from the shore to the study site to contribute sizably to DMS production in the water column as they sink and degrade during the senescence phase. However, we are yet to address its loss/emission processes across the oxic-hypoxic boundary of seasonal (and permanent) oxygen minimum zone of the northern Indian Ocean, which is important from the viewpoint of global climate change.

Key words: Arabian Sea, DMS, DMSP, hypoxic, macroalgae, phytoplankton

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Introduction

The Arabian Sea is one of the five major upwelling zones of the world ocean and experiences large

intra-annual variability due to extreme atmospheric forces as compared to the world oceans (Kumar

et al., 2000). During the southwest monsoon (SWM), in the Arabian Sea strong upwelling is

experienced along the western boundary (Somalia-Arabia coast) compared to the eastern part (west

coast of India), which experiences weaker upwelling (Banse, 1959; 1968; Ahmed et al., 2017).

Though weaker, this coastal upwelling brings cold nutrient-rich subsurface water into the euphotic

zone resulting in high biological production (Banse, 1968; 1987; Sankaranarayanan et al., 1978;

Naqvi et al., 2000; Wiggert et al., 2005; Habeebrehman et al., 2008, Ayaz et al., 2017), with diatoms

as the dominant phytoplankton group (Krey, 1973; Ahmed et al., 2017). On the other hand, during

nutrient deplete condition that occurs in April and May (spring intermonsoon, SIM) supports

Trichodesmium sp. (Devassy et al., 1978; Sarangi et al., 2004) along the west coast of India.

Marine phytoplankton are known to produce dimethylsulphoniopropionate (DMSP), a

precursor for dimethylsulphide (DMS), an anti-greenhouse gas. DMSP in phytoplankton plays a

critical role as osmoregulator, cryoprotectant and as an antioxidant (Barnard et al., 1984; Turner et

al., 1988; Malin et al., 1993; Matrai and Keller, 1993; Pandey et al., 2012). As reported by Liss et al.

(1993), different phytoplankton groups are known to produce varying concentration of DMSP which

follows the following order: Coccolithophores > Phaeocystis > Dinoflagellates > Diatoms.

Breakdown of DMSP by DMSP-lyase, which is found in some algae (Steinke et al., 2007) and

marine bacteria (Kiene, 1992) is considered to be the main source of DMS. Also, biological

processes such as zooplankton grazing, viral attack, aging and dying of the phytoplankton cell can

accelerate degradation of DMSP to DMS process (Dacey and Wakeham, 1986; Kwint and Krammer,

1995; Pandey et al., 2012). Furthermore, various anoxic environments can produce DMS possibly

through microbial methylation of methanethiol which is derived from H2S methylation or methionine

catabolism (Kienne and Hines, 1995; Stets et al., 2004; Carrion et al., 2015). Such low oxygen

condition is known to occur on the shelf along the west coast of India (Naqvi et al., 2000). Thus,

DMS is found abundantly in sea water, contributing approximately to two-thirds of the global marine

sulphur flux from the marine environment (Andreae, 1990).

DMS released in the atmosphere gets photo-oxidised producing non-seasalt sulphate aerosols,

which acts as cloud condensation nuclei and leads to the reflection of solar incoming radiation

(Charlson et al., 1987). On the other hand, oxidation of DMS also contributes to the acidity of

rainfall (Nguyen et al., 1992). Thus, due to the linkage between biological production of DMS,

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atmospheric aerosols and cloud albedo, DMS is gaining great research interest (Charlson et al.,

1987).

Thus, the present study addresses the seasonal changes in physicochemical parameters and its

impact on phytoplankton community structure and production of DMSP and DMS at a shallow (~28

m) coastal station (CaTS G5) located off Goa, along the central west coast of India.

Materials and Methods

Study area and sampling

Subsurface waters over the western Indian continental shelf experiences seasonal

hypoxia/suboxia/anoxia towards the end of SWM every year (Naqvi et al., 2006). However, the

intensity of low oxygen conditions varies from year to year. The data presented here was collected

from the Candolim Time Series (CaTS) - G5 station (Lat: 15º30.693’N, Long: 73º39.065’E).

Sampling was done during the north east monsoon (NEM, December and January), SIM, April), late

SWM (September) and fall inter-monsoon (FIM, October and November) during the year 2015 (Fig.

1). The data presented in graphical form is taken as average for the two months for NEM and FIM .

Water samples were collected using a 5 L Niskin sampler. Subsampling was done for dissolved

oxygen (DO), DMS, total DMSP (DMSPt), nutrients (NO3, NO2, PO4 and SiO2), chlorophyll (Chl) a

and phytoplankton (taxonomy and enumeration). During gas sampling (DO and DMS) enough care

was taken to avoid any atmospheric exchange. Temperature and salinity were measured using

sensors fitted to the portable CTD system (SBE 19plus V2 SeaCAT Profiler). DO was measured by

Winkler’s titration method (Grasshoff et al., 1983). Samples for nutrients were analysed using a

SKALAR autoanalyser.

Chlorophyll a

Samples for Chl a estimation were collected in amber colored 1 L plastic bottles and

transported to the laboratory at low temperature. At the shore laboratory, 500 mL of water later

filtered onto a 47 mm GF/F filter paper (0.7 μm) and the Chl a pigment was extracted in 10 ml of

90% acetone in the dark for 24 h at -20°C (UNESCO, 1994). The fluorescence was measured using a

fluorometer (Turner Designs, AU-10, CA, USA).

Phytoplankton taxonomy and enumeration

Phytoplankton samples (250 mL) were collected in amber coloured plastic bottles and fixed

with Lugol’s Iodine (1% v/v). These samples were allowed to settle for 48 hours and concentrated to

a known volume by carefully siphoning the top layer of the water sample using a silicon tubing, one

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end of which was covered with 11 µm Nylon mesh. By using a Sedgwick rafter chamber, 1 mL of

each concentrate was analysed using an inverted Olympus microscope (IX51) at 200X

magnification. Phytoplankton cells were identified by using standard taxonomic key (Tomas, 1997)

and cell abundance is expressed in cells L-1.

DMS and DMSP measurements

Samples for DMS and DMSPt were collected in 60 mL amber-colored ground glass stoppered

bottles. The samples were preserved in dark at 4ºC until analysis. However, the storage time never

exceeded 4 h. The analyses were carried out using a Shimadzu Gas Chromatograph (GC 2010) fitted

with a Flame Photometric Detector (FPD). DMS and DMSPt were analysed according to the method

given by Turner et al. (1990), which is also detailed in Shenoy et al. (2002). Nitrogen gas was used

to purge a measured volume of seawater sample. The stripped sulphur gases were then dried using

three different moisture traps and separated on Chromosil 330 column and detected by the FPD.

Following DMS measurement, DMSP was hydrolysed to DMS by adding 1 mL of 10 M NaOH to

the same samples with immediate purging for 20 min. This results in cleavage of DMSP into DMS

and acrylic acid. Tests revealed close to 100 % conversion to DMS from DMSPt. Calibration was

performed using DMSP standard (Research Plus, New Jersey). The precision of this method was

better than 5%.

Laboratory experiment with Sargassum sp.

Rocky shore along the central west coast of India (all along the west coast of India in general)

is sizably populated by seaweeds. We hypothesize, this seaweed might contribute to DMSP and

DMS production at the study site while they are washed offshore. Therefore, to understand their role

in DMSP-DMS production, floating Sargassum sp. at the study site were collected and used for

experimental purpose. Sargassum sp. healthy leaf (Avg. wet weight 0.11 + 0.1 gm) was collected

from study location and incubated in 30 mL glass serum bottles containing field water (2 blanks and

3 experimental) at room temperature (avg. 27.5˚C, a temperature close to average annual surface

water temperature at the study site) for a week. Blank corrected values are presented and discussed

below.

Hydrogen sulphide analysis

5 mL of water sample was taken from the experimental bottle by using 20 mL syringe, to

which 0.4 mL of diamine reagent was added. Further, the sample was mixed gently and analysed

spectrophotometrically (Shimadzu, UV 1700 Spectrophotometer) following Cline (1969).

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Results and discussion

The seasonal study carried out at CaTS location showed wide variation in physicochemical

parameters having an impact on phytoplankton and DMS production. During the study period,

considerable variations in phytoplankton abundance and diversity was recorded, which was linked

with water column properties like stratification/light/temperature/turbulence/nutrient

availability/zooplankton grazing (Dawning, 1997; Acharyya et al., 2012; Joint and Pomroy, 1981;

Pennock and Sharp, 1994; Baliarsingh et al., 2015).

Hydrography

Temperature

During the study period, the entire water column temperature ranged from 24 to 31°C (Avg. 28.4°C).

Overall surface water temperature that varied from 28 to 31°C (Avg. 29.4°C) was about 2°C warmer

than the bottom water temperature (24 to 31°C; Avg. 27.5°C, Fig.2). During the SWM, subsurface

and near-bottom waters were colder as compared to the surface with a clear signature of upwelling.

At times, a similar condition was found to prevail even during the FIM. While, water column during

NEM remained stratified which may be due to intrusion of low saline waters (see below) from BoB

(Jyothibabu et al., 2008) as seen from uniform water column temperature (28ºC) in upper strata (0 to

18 m) and slightly lower temperature (27.1ºC) at near bottom (27 m). On the other hand, the

maximum temperature (31ºC) during SIM may be linked to the high incoming solar insolation during

this period (Shenoy and Patil, 2003).

Salinity

The salinity of the entire water column varied between 32 and 36 (Avg. 34.2, Fig.3).

Maximum salinity (36) was recorded in the subsurface waters. Low saline waters (34.5) recorded at

the near surface during monsoon (SWM) and in FIM suggests the influence of freshwater either from

rain (Jayaraman and Gogate, 1957) or riverine discharge (Devassy and Goes, 1988) and in NEM due

to the influence of Bay of Bengal water (Jyothibabu et al., 2008). High salinity near the bottom

during SWM (associated with low temperature) is a signature of the upwelling phenomenon.

Dissolved oxygen

DO concentration in the water column (0-27 m) varied from 0.2 to 6.6 mg L-1 (Avg. 5.2 mg

L-1, Fig.4). Maximum DO was recorded during SIM while very low DO concentration was observed

near bottom during the SWM. During the entire study period, DO in the surface water ranged from

5.8 to 6.6 mg L-1 (Avg. 6.2 mg L-1) whereas the bottom water DO concentration varied from 0.2 to

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6.1 mg L-1 (Avg. 3.8 mg L-1). Low oxygen concentration recorded during the SWM may be

attributed to the seasonal upwelling experienced along the southwest coast of India (Naqvi et al.,

2000).

Nutrients

Nutrients such as NO3, NO2, PO4 and SiO2 concentrations varied from 0.1 to 13 µM (Avg.

2.2 µM), undetectable to 3.4 µM (Avg. 0.4 µM), 0.1 to 1.8 µM (Avg. 0.5 µM) and 3.5 to 34.9 µM

(Avg 10.4 µM), respectively (Fig.5). Annually, within the water column, the highest concentration of

nutrients was recorded during the SWM. As compared to surface and mid-depth waters, high NO3,

NO2, SiO2 and PO4 concentration was recorded near the bottom depths. While, degradation of

organic matter at near-bottom depths, perhaps contribute more during non-SWM period. Overall,

nutrient contribution decreased seasonally from SWM to FIM, NEM and SIM season.

Chlorophyll a

Chl a concentration of the water column varied from 0.4 to 1.3 µg L-1 (Avg. 0.79 µg L-1,

Fig.6). Surface water Chl a concentration that varied from 0.4 to 0.9 µg L-1 (Avg. 0.68 µg L-1), was

relatively lower than bottom water (0.6 to 1.1 µg L-1; Avg. 0.84 µg L-1). Maximum Chl a

concentration (1.31 µg L-1) was recorded at mid-depth (9 m) during the FIM. Recorded high Chl a

concentration was supported by high phytoplankton abundance, which may be due to utilization of a

load of nutrients brought in during the SWM and favorable environmental conditions in terms of

light availability for proliferation of diatoms and dinoflagellates during FIM (Devassy and Goes,

1988; Subrahmanyan, 1959; Qasim et al., 1972, Baliarsingh et al., 2015). Apart, the experimental

study by Qasim et al. (1972) showed that waters with reduced salinity (within limits) supported

greater phytoplankton abundance, a condition of low saline nutrient-rich waters that prevails during

FIM may be yet another reason for high phytoplankton abundance and high Chl a concentration. In

contrast, low Chl a concentration and low phytoplankton abundance recorded during SIM was due to

the oligotrophic nature of water column (Jyothibabu et al., 2008) with low NO3 concentration (0.1 -

0.5 µM) in the study area.

Distribution of DMS and DMSPt

Concentrations of DMS and DMSPt in the water column varied from 4 to 116 nM (Avg. 20.5

nM) and 1 to 11 nM (Avg. 7.71 nM), respectively (Fig.7). The maximum concentration of DMS (116

nM) was recorded at 18 m depth during the SWM, which may be due to formation of DMS in oxic-

hypoxic/suboxic waters (see below). The DMS concentration in the surface water during the entire

study period ranged from 9 to 25 nM (Avg. 18.42 nM), whereas the surface DMSPt concentrations

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ranged from 5 to 11 nM (Avg. 8.36 nM.). DMS and DMSPt concentration in the bottom waters

varied from 4 to 12 nM (Avg. 6.89 nM) and 6 to 8 nM (avg. 6.73 nM), respectively. Maximum

average DMS and DMSPt was observed during the SWM followed by SIM, NEM and FIM.

Phytoplankton community structure

During the study period, phytoplankton abundance in the water column varied from 8.6*103

to 94*103 Cells L-1 (Avg. 37.1*103 Cells L-1, Fig.8). Maximum phytoplankton abundance (94*103

Cells L-1) was recorded at subsurface (9 m) during NEM. However, phytoplankton abundance

observed in the surface water that varied from 8.6*103 to 63*103 Cells L-1 (Avg. 35.8*103 Cells L-1)

was not very different from bottom water (9*103 to 54*103 Cells L-1; Avg. 37.8*103 Cells L-1).

Seasonally, high abundance of phytoplankton (Avg. 64.7*103 Cells L-1) was observed in FIM

followed by SWM (Avg. 49.7*103 Cells L-1), SIM (Avg. 18.4*103 Cells L-1) and NEM (Avg.

15.4*103 Cells L-1). On an average, (annual scale) phytoplankton community was largely dominated

by diatoms (84%) followed by dinoflagellates (16 %). Amongst, 91 species of phytoplankton, 60

genera were of diatoms (39 species: 21 centric and 18 pennate) and 21 genera of dinoflagellates.

Chaetoceros spp., Nitzschia spp., Pronoctiluca spp. and Scrippsiella spp. were the dominant forms.

Seasonal variation

Seasonal study showed maximum phytoplankton abundance and diversity during FIM

followed by SWM, SIM and NEM, while the maximum concentration of DMS was recorded during

SWM followed by SIM, NEM and FIM. During SWM, five times higher concentration of DMS was

recorded in the subsurface waters associated with high phytoplankton abundance and high Chl a

concentration. Recorded high DMS concentration may be due to increase in phytoplankton

productivity and associated processes driven by coastal upwelling (Shenoy and Kumar, 2007).

However, compared to FIM, phytoplankton abundance was low in SWM which may be due to heavy

rainfall during this period resulting in massive quantities of freshwater input leading to extensive

dilution and stratification of the water column. Also, the cloud cover and high turbidity of the water

during SWM results in restricted light penetration in the water column. Due to which, no effective

utilization of the nutrients brought about by land run off in absence of conducive environmental

conditions, resulting in a low phytoplankton density (Sankarnarayanan and Qasim, 1969; Devassi

and Goes, 1988; Baliarsingh et al., 2015). However, few forms such as Rhizosolenia spp, Nitzschia

spp. and Scrippsiella spp. acclimatized to this condition dominated and contributed the maximum to

the total phytoplankton population during this period.

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High phytoplankton and low DMS during FIM showed species specificity and the role of

physicochemical parameters regulating their abundance and distribution. High concentration of DMS

and DMSPt was recorded in the surface waters with a decreasing trend towards the bottom. High

nutrient availability brought about by SWM and conducive environment (reduced cloud cover, low

turbidity and improved light penetration) during the FIM may be the reason for high phytoplankton

abundance and diversity.

As compared to SWM, low DMS, DMSPt, phytoplankton abundance and high phytoplankton

diversity was recorded during NEM. Recorded lows believed to be due to the resulting intrusion of

low saline waters from Bay of Bengal leading to stratified condition (BOB; Sanilkumar et al., 2003;

Jyothibabu et al., 2008). Thus providing optimum environmental conditions which are probably the

prime factors controlling the phytoplankton growth and proliferation of the majority of species

(Qasim et al., 1972; Devassy and Goes, 1988) resulting diverse phytoplankton community in NEM.

Similarly, low nutrients during the SIM period is attributed for low phytoplankton population

and low Chl a concentration in the water column except during period of Trichodesmium blooms that

were mainly restricted to the surface water. However, few phytoplankton with low nutrients and

high-temperature requirement are known to flourish well in the region (Devassy and Goes, 1988).

Thalassiosira spp., Pleurosigma spp. and Scrippsiella spp. were the most dominant and contributed

the maximum to the total phytoplankton density during SIM season. Observed high DMS

concentration during SIM may be due to intensive grazing activity by zooplankton, as the

zooplankton production in the study region is reported to be high during SIM (Nair, 1980; Pant et al.,

1984). Levasseur et al. (1996) and Wolfe and Steinke (1996) also reported a similar trend in their

study region. While, recorded high average DMS concentration in the subsurface region during

SWM and NEM is thought to be due to the changes in the biogeochemical processes brought about

by upwelling during SWM (Wiggert et al., 2005) and intrusion of low saline waters from BOB

leading to stratified condition along the west coast of India in NEM (Sanilkumar et al., 2003;

Jyothibabu et al., 2008). During SWM, high DMS and low DMSPt concentration in the subsurface

region revealed that phytoplanktons may not be the major contributors directly for high DMS

concentration. Instead, high DMS in the subsurface region could be due to prevailing low oxygen

condition. The very low levels of dissolved oxygen (particularly anoxic) is known to influence the

dynamics of DMS – enhancement of its production on one side and decreased bacterial consumption

on the other hand (See Omori et al., 2015) believed to have led to build-up of DMS at the study site.

Apart, we also consider that prevailing low oxygen condition (with varying intensity year to year:

hypoxic/suboxic /anoxic) well within the euphotic zone supported redox transformations, for

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example, in trace metals like iron reduction (Fe+3 Fe+2). Bioavailable form of iron (Fe+2) is known

to play an important role in enzymatic reaction of phytoplankton growth. This together with the

prevailing high nutrient level (NO3: ~13µM) possibly supported growth of autotrophs as seen from

previously measured high rates of primary productivity (surface:~10 mgCm-3 d-1 & subsurface

(18m): ~100 mg C m-3 d-1) and Chl a biomass (~1 µg L-1) at mid-depth. The observed high DMS in

low oxygen subsurface waters (oxic-hypoxic/suboxic) is thus attributed to the higher bioavailable

iron under reduced condition at the study site. This is in consistent with the findings of iron

fertilization experiments where a sizable increase in DMS concentration was recorded within the

purposefully added iron fertilized patch (Turner et al., 2004; Liss et al., 2004, Gunson et al., 2006).

Thus, we consider seasonally occurring reduced condition along the west coast of India naturally

supports bioavailable iron thereby enhancing phytoplankton growth and DMS production. Apart

from algal derived organosulphur precursor, significant amount of DMS is known to be produced

through methylation of thiols in anaerobic and aerobic habitats (Fig. 9A; Drotar et al., 1987; Finster

et al., 1990; Stets et al., 2003; Schafer et al., 2010). Formation of DMS by methylation which occurs

via transfer of methyl (CH3) group to the acceptor (methanethiol) that can originate either from

bimethylation of H2S or by degradation of organosulphur compounds (Zinder and Brock, 1978;

Kiene and Capone, 1988; Stets et al., 2003). High DMS concentration associated with low DMSPt

during the SWM in the present study may be due to the wash away of seaweeds mostly Sargassum

sp. at the study site; seaweeds are found all along the western coast of India. According to the study

carried out by Thakur et al. (2008) and Pereira and Almeida (2014), most of the Sargassum sp. along

the western coast of India complete their life cycle in May-June and in November-December and

later gets detached from their natural habitat due to senescence and turbulence/wave action.

Degradation and sinking of these seaweeds perhaps contributed to DMS (and H2S) in the water

column. To confirm this, a laboratory experiment was conducted wherein healthy floating Sargassum

sp. was incubated in surface water at shore laboratory maintaining experimental bottle temperature

similar to that at field (27.5ºC). The DMS and DMSPt concentration was measured over a period of 5

days, where increase in DMS concentration was recorded linear increase from day 1 (559 nM) to day

3 (621 nM) which was later masked by H2S peak in the subsequent analysis (Fig. 10). While, a

decrease in DMSPt concentration from 670 nM (day 1) to 605 nM (day 4) signifies microbial

degradation of DMSP to DMS (Kiene, 1992).

Whereas high subsurface averaged DMS concentration during NEM coincide well with high

averaged DMSPt associated with low Chl a concentration, low phytoplankton abundance and

diversity. This suggests that the DMS in the subsurface region is phytoplankton (mostly

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dinoflagellates) derived (Fig. 9B) and is species specific. The specific species during the NEM

present in the subsurface region (11% diatoms and 30 % dinoflagellates; Chaetoceros cuvisetus,

Dactyliosolen fragillisimus, Entomoneis spp., Gyrosigma spp., Licmophora spp., Dictyocha spp.,

Dinophysis miles, Ornithocercus magnificus, Ornithocercus spp., Podolampas palmipes and

Pronoctiluca spp.) may be the contributors for the observed high subsurface DMS and DMSPt

concentration.

Conclusions

The present study reveals that prevailing low oxygen condition along the west coast of India is very

dynamic from the view point of ocean-atmosphere trace gas exchanges not only in nitrous oxide

production (Naqvi et al., 2000) but also in DMS build-up. Furthermore, our laboratory experiment

gave a clue for unknown source of subsurface high DMS concentration (see also Shenoy et al., 2012)

during SWM, that the prevailing floating Sargassum sp. and its sinking and degradation along with a

load of organic matter brought about by riverine discharge together with methylation pathway seem

to contribute statistically to subsurface high DMS. Prevailing oxic-hypoxic (/suboxic/anoxic)

boundary seem to support DMS build up in the region. However, we are yet to address its rates of

production/loss/emission processes across the oxic-hypoxic boundary of seasonal (and permanent)

oxygen minimum zone of the northern Indian Ocean, which is important from the viewpoint of

global climate change.

Acknowledgment

We wish to thank the Director, CSIR-National Institute of Oceanography for providing the

facilities to undertake this work. We also thank Dr. Sai Elangovan for his help in statistical analysis.

The financial assistance from the Council of Scientific & Industrial Research and the Ministry of

Earth Sciences, Government of India, under projects GAP 2424 and OLP 1707 is gratefully

acknowledged.

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Figures

 

 

Fig.1. Map showing the Candolim time series (CaTS, G5) station located off Goa, central west coast of India.

 

Fig.2. Seasonal variation of temperature (ºC) at G5 location.

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Fig.3. Seasonal variation of salinity at G5 location.

 

Fig.4. Seasonal variation of Dissolved oxygen (DO; mg L-1) at G5 location.

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Fig.5. Seasonal variation of NO3, NO2, PO4 and SiO2 (µM) at G5 location.

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Fig.6. Seasonal variation of Chl a (µg L-1) at G5 location.

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Fig.7. Seasonal variation of DMS (nM) and DMSPt (nM) at G5 location.

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Fig.8. Seasonal variation of phytoplankton abundance (No. of organisms*103 Cells L-1) at G5 location.

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Fig.9A and 9B. Schematic representation of DMS pathways following Schafer et al., 2010. Fig.9A. indicates 

DMS pathways during SWM season, where  line‐graph  (x‐axis)  indicates  the oxygen concentration  in  the 

water column. Fig.9B. Indicates DMS pathways during NEM season when the water column in fully oxic. 

 

 

 

 

 

 

 

 

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Fig.10. DMS, DMSPt and H2S production during the incubation experiment at laboratory condition.

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Plate 1. Incubation experiment on Sargassum sp. at laboratory condition showing one control bottle (control) with

no Sargassum sp. (leaf) and 3 experimental bottles (Exp 1, 2, 3) with Sargassum sp. (leaf).

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Plate 2. Formation of H2S in experimental bottle from day 3 to day 5. Day 3 sample shows light coloration as

compared to day 4 and 5 indicating the presence of low H2S on day 3 and subsequent build up on day 4 and 5.